The invention relates to anode materials containing silicon particles, the use thereof for producing lithium ion batteries and also the lithium ion batteries obtainable in this way.
Rechargeable lithium ion batteries are today the practical electrochemical energy stores having the highest gravimetric energy densities of, for example, up to 250 Wh/kg. They are used first and foremost in the field of portable electronics, for tools and also for electrically powered transport means, for example bicycles or automobiles. However, especially for use in automobiles, it is necessary to achieve a further significant increase in the energy density of the batteries in order to attain longer electric ranges of the vehicles.
At present, graphitic carbon is widespread as material for the negative electrode (“anode”) of lithium ion batteries. During incorporation and release of lithium, i.e. during charging and discharging of the battery, graphitic carbon advantageously experiences only small volume changes, for example in the region of 10% for the limiting stoichiometry of LiC6. However, a disadvantage is its relatively low electrochemical capacity of theoretically not more than 372 mAh per gram of graphite, which corresponds to only about one tenth of the electrochemical capacity which is theoretically achievable when using lithium metal.
For this reason, there has for a long time been a search for alternative materials for anodes, especially in the field of (semi)metals which form alloys with lithium. A challenge here is frequently the large volume expansion during incorporation or release of lithium into/from the alloy-forming (semi)metals, which is more than 200%, sometimes even up to 300%.
Silicon forms binary electrochemically active alloys with lithium and these can have a very high lithium content. The known maximum lithium content is found in Li4.4Si, which corresponds to a very high theoretical specific capacity in the region of 4200 mAh per gram of silicon. Disadvantageously, the incorporation and release of lithium into/from silicon is associated with a very large volume change which can attain 300%. This volume expansion leads to high mechanical stressing of the crystallites which ultimately leads to them breaking apart. This process, which is referred to as electrochemical milling, leads to a loss of electrical contacting in the active material and in the electrode structure and thus to destruction of the electrode with a loss of capacity. The decrease in the capacity during the course of a number of charging and discharging cycles is referred to as fading or continuous capacity decrease and is generally irreversible. For example, mixtures of micron-scale silicon and carbon give corresponding electrodes having very high initial capacities of more than 2000 mAh/g, but these suffer from pronounced fading.
To reduce the mechanical stress and thus to prevent electrochemical milling, the use of nanosize silicon particles for anodes of lithium ion cells has frequently been recommended. With regard to size and shape of the nanosize silicon particles, the teaching goes in a variety of directions in the literature. Thus, anode materials based on nanosize or nanostructured silicon particles having average particle sizes of, for example, <0.2 μm have frequently been described. EP 1730800 teaches electrode material for lithium ion batteries which contains nanosize silicon particles which have average primary particle diameters of from 5 to 200 nm and are joined together to form aggregates having sizes of up to 1000 nm. WO 2014/202529, too, recommends nanosize silicon particles for electrode material for lithium ion batteries having volume-weighted particle size distributions between the diameter percentiles d10>20 nm and d90<2000 nm. For example, d10 values of 80 nm and 120 nm and also d50 values of 190 nm and 150 nm are specifically disclosed.
In a Journal of Power Sources, 256, 2014, pages 32 to 36, M. Gauthier discusses anode materials comprising silicon particles which are characterized by very broad, multimodal size distributions and comprise, for example, nanosize particles in the region of 40 nm and considerable proportions of coarse particles having diameters of >>30 μm. The half cells described do not yet achieve the coulombic efficiency required in practice. Electrode materials containing coarse silicon particles are known, for example, from US 2003235762, and these have a content of silicon particles having particle diameters of from 1 to 10 μm of at least 60% by volume. US 2003235762 says nothing about the production process for the silicon particles and thus also does not implicitly disclose the particle shape or sphericity of the silicon particles and in particular no unaggregated silicon particles. Aggregated and unaggregated silicon particles are not distinguishable by means of the static light scattering usually used for determining particle diameters. The active material of US 2009305129 contains silicon particles having crystallite sizes of <100 nm and particle sizes of from 7 to 25 μm, which are produced by gas-phase processes. The silicon particles of the lithium ion batteries of US 2005/0214646 have average particle diameters of 1 μm. US 2005/0214646 is silent about the particle size distribution and the production of the silicon particles. US 2005/0214646 advises a lithium/silicon ratio of not more than 4.0 for the anode material of the charged batteries. Molar Li/Si ratios of, for example, 3.5 and above are specifically described. JP 4911835 teaches Li/Si ratios for the anode material of charged lithium ion batteries in the range from 2.3 and 4.0.
Previously known nanosize silicon particles continue to lead to high initial and continuous capacity losses in lithium ion batteries. The reason for this is the volume change experienced by the silicon particles during charging and discharging of the lithium ion batteries and the associated mechanical attrition of the anode. In addition, the surface of the silicon anode material reacts with constituents of the electrolyte with continuous formation of passivating protective layers (solid electrolyte interface; SEI), which leads to immobilizing of lithium. Due to the volume change experienced by the silicon, these passivating protective layers are only partially stable, so that a certain amount of lithium is immobilized during each charging/discharging cycle. Since the amount of mobile lithium in the full cell, which corresponds to the usable capacity, is limited by the cathode material, this is quickly consumed and the capacity of the cell is decreased after too few cycles. The decrease in the reversible capacity of lithium ion batteries during continuing cycles is also referred to as fading.